Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()

From: Mikulas Patocka <mpatocka@redhat.com>
To: Dmitry Safonov <dima@arista.com>
Cc: linux-kernel@vger.kernel.org,
	"Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Daniel Axtens" <dja@axtens.net>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Michael Neuling" <mikey@neuling.org>,
	"Nathan March" <nathan@gt.net>, "Pasi Kärkkäinen" <pasik@iki.fi>,
	"Peter Hurley" <peter@hurleysoftware.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Rong, Chen" <rong.a.chen@intel.com>,
	"Sergey Senozhatsky" <sergey.senozhatsky.work@gmail.com>,
	"Tan Xiaojun" <tanxiaojun@huawei.com>,
	"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
	stable@vger.kernel.org,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jslaby@suse.com>, "Jiri Slaby" <jslaby@suse.cz>,
	syzbot+3aa9784721dfb90e984d@syzkaller.appspotmail.com
Subject: Re: [PATCHv5 0/7] tty: Hold write ldisc sem in tty_reopen()
Date: Wed, 19 Sep 2018 16:03:41 -0400 (EDT)	[thread overview]
Message-ID: <alpine.LRH.2.02.1809191602230.16174@file01.intranet.prod.int.rdu2.redhat.com> (raw)
In-Reply-To: <1537379148.21380.16.camel@arista.com>

On Wed, 19 Sep 2018, Dmitry Safonov wrote:

> On Wed, 2018-09-19 at 13:35 -0400, Mikulas Patocka wrote:
> > 
> > On Wed, 19 Sep 2018, Mikulas Patocka wrote:
> > 
> > > 
> > > 
> > > On Tue, 18 Sep 2018, Dmitry Safonov wrote:
> > > 
> > > > Hi all,
> > > > 
> > > > Three fixes that worth to have in the @stable, as they were hit
> > by
> > > > different people, including Arista on v4.9 stable.
> > > > 
> > > > And for linux-next - adding lockdep asserts for line discipline
> > changing
> > > > code, verifying that write ldisc sem will be held forthwith.
> > > > 
> > > > The last patch is an optional and probably, timeout can be
> > dropped for
> > > > read_lock(). I'll do it if everyone agrees.
> > > > (Or as per discussion with Peter in v3, just convert ldisc to
> > > > a regular rwsem).
> > > > 
> > > > Thanks,
> > > > Dima
> > > 
> > > I confirm that this patch series fixes the crash for me.
> > > 
> > > Tested-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > I was too quick to acknowledge this patchset. It doesn't work.
> > 
> > This patchset fixes the crash, but it introduces another bug - when I
> > type 
> > 'reboot' on the console, it prints 'INIT: Switching to runlevel: 6'
> > and 
> > then it gets stuck for 80 seconds before proceeding with the reboot.
> > When 
> > I revert this patchset 'reboot' reboots the machine without any
> > delay. 
> > This bug was reproduced on Debian 5 userspace on pa-risc.
> 
> Thanks much for the testing, Mikulas.
> Could you try to bisect which of the patches causes it?
> The most important are 1,2,3 - probably, one of them caused it..
> I'll stare a bit into the code.

The patch 3 causes it.

The hangs during reboot take either 80 seconds, 3 minutes or 3:25.

Mikulas